Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752671AbdFHAIY (ORCPT ); Wed, 7 Jun 2017 20:08:24 -0400 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:50130 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752116AbdFGXCT (ORCPT ); Wed, 7 Jun 2017 19:02:19 -0400 From: Willy Tarreau To: linux-kernel@vger.kernel.org, stable@vger.kernel.org, linux@roeck-us.net Cc: NeilBrown , Jens Axboe , Willy Tarreau Subject: [PATCH 3.10 023/250] block_dev: don't test bdev->bd_contains when it is not stable Date: Thu, 8 Jun 2017 00:56:49 +0200 Message-Id: <1496876436-32402-24-git-send-email-w@1wt.eu> X-Mailer: git-send-email 2.8.0.rc2.1.gbe9624a In-Reply-To: <1496876436-32402-1-git-send-email-w@1wt.eu> References: <1496876436-32402-1-git-send-email-w@1wt.eu> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2584 Lines: 66 From: NeilBrown commit bcc7f5b4bee8e327689a4d994022765855c807ff upstream. bdev->bd_contains is not stable before calling __blkdev_get(). When __blkdev_get() is called on a parition with ->bd_openers == 0 it sets bdev->bd_contains = bdev; which is not correct for a partition. After a call to __blkdev_get() succeeds, ->bd_openers will be > 0 and then ->bd_contains is stable. When FMODE_EXCL is used, blkdev_get() calls bd_start_claiming() -> bd_prepare_to_claim() -> bd_may_claim() This call happens before __blkdev_get() is called, so ->bd_contains is not stable. So bd_may_claim() cannot safely use ->bd_contains. It currently tries to use it, and this can lead to a BUG_ON(). This happens when a whole device is already open with a bd_holder (in use by dm in my particular example) and two threads race to open a partition of that device for the first time, one opening with O_EXCL and one without. The thread that doesn't use O_EXCL gets through blkdev_get() to __blkdev_get(), gains the ->bd_mutex, and sets bdev->bd_contains = bdev; Immediately thereafter the other thread, using FMODE_EXCL, calls bd_start_claiming() from blkdev_get(). This should fail because the whole device has a holder, but because bdev->bd_contains == bdev bd_may_claim() incorrectly reports success. This thread continues and blocks on bd_mutex. The first thread then sets bdev->bd_contains correctly and drops the mutex. The thread using FMODE_EXCL then continues and when it calls bd_may_claim() again in: BUG_ON(!bd_may_claim(bdev, whole, holder)); The BUG_ON fires. Fix this by removing the dependency on ->bd_contains in bd_may_claim(). As bd_may_claim() has direct access to the whole device, it can simply test if the target bdev is the whole device. Fixes: 6b4517a7913a ("block: implement bd_claiming and claiming block") Signed-off-by: NeilBrown Signed-off-by: Jens Axboe Signed-off-by: Willy Tarreau --- fs/block_dev.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/fs/block_dev.c b/fs/block_dev.c index 85f5c85..3a85a03 100644 --- a/fs/block_dev.c +++ b/fs/block_dev.c @@ -655,7 +655,7 @@ static bool bd_may_claim(struct block_device *bdev, struct block_device *whole, return true; /* already a holder */ else if (bdev->bd_holder != NULL) return false; /* held by someone else */ - else if (bdev->bd_contains == bdev) + else if (whole == bdev) return true; /* is a whole device which isn't held */ else if (whole->bd_holder == bd_may_claim) -- 2.8.0.rc2.1.gbe9624a